Mobile computing devices, such as notebook PCs, smart phones, and tablet computing devices, are now common tools used for producing, analyzing, communicating, and consuming data in both business and personal life. Consumers continue to embrace a mobile digital lifestyle as the ease of access to digital information increases with high-speed wireless communications technologies becoming ubiquitous. Popular uses of mobile computing devices include displaying large amounts of high-resolution computer graphics information and video content, often wirelessly streamed to the device.
While these devices typically include a display screen, the preferred visual experience of a high-resolution, large format display cannot be easily replicated in such mobile devices because the physical size of such device is limited to promote mobility. Another drawback of the aforementioned device types is that the user interface is hands-dependent, typically requiring a user to enter data or make selections using a keyboard (physical or virtual) or touch-screen display.
As a result, consumers are now seeking a hands-free high-quality, portable, color display solution to augment or replace their hands-dependent mobile devices.
One example of such a display solution is the active matrix light emitting diode (LED) display. The active matrix LED display uses a storage capacitor, for each pixel, that is charged by a driving voltage during a display scan period. The capacitor stores the voltage until the next scan frame, at which time the capacitor stores a new voltage corresponding to that scan frame. The stored voltage provides a reference to the pixel circuit for driving current to LED during the one frame time—the amount of current driven depends on the value of the stored voltage.
For the example active matrix LED display shown in FIG. 1, each unit pixel consists of transistors 1, 2 and 4, a capacitor 3, and an LED 5. The gate of the transistor 1 receives a select signal through a Select Line (SL) while its source receives a voltage data signal through a VData line. The voltage data signal is transmitted to the gate of the transistor 2 when the transistor 1 is turned on by the select signal and the voltage level of the data signal VData turns on the transistor 2 to generate a driving current through the transistor 2 lighting the LED 5 during the transistor 4 turn on time.
A disadvantage of the circuit depicted in the example of FIG. 1 is that the output of the LED driving circuit (i.e., the LED driving current) may be sensitive to circuit parameter variations. Such parameter variations may include, for example, variations of the transistors' threshold voltages, and variations in the widths and lengths of the transistor physical gate geometries. The differences between the driving currents from pixel to pixel may lead to non-uniform illumination on the active matrix LED display.